This invention concerns a method for producing high-strength, thermally-stable, silicon carbide (SiC) fibers with low boron contents. The method is also applicable to the fabrication of other SiC ceramics, such as SiC coatings, thin substrates, porous ceramics, etc., with low boron content. In general, the invention relates to treatment of the SiC fibers after sintering to remove residual boron, and in particular relates to exposing the SiC fibers to an atmosphere of CO gas under elevated temperature.
Silicon carbide (SiC) is a material with excellent mechanical properties at high temperatures. In order to obtain high strength, it is important to produce SiC ceramics with high relative density (i.e., low residual porosity) and fine grain sizes. However, it is difficult to prepare pure SiC with high relative density and fine grain sizes by sintering methods, especially by pressureless sintering methods. In samples comprised of fine particles or fine crystallites, pure SiC generally undergoes coarsening or growth of crystallites and pores during high temperature heat treatment because of the dominance of surface diffusion and/or vapor phase diffusion processes. Thus, very little densification from pore removal occurs in pure SiC during high temperature heat treatment. As a result of this problem, sintering aids are used to enhance densification and to prevent coarsening during sintering and thereby allow the fabrication of SiC with high relative density and fine grain sizes. Several additives have been found effective as sintering aids for SiC, but boron-containing compounds are the most commonly used additives. Varying amounts of boron compounds have been reported as effective for sintering (e.g., 0.2-5 wt %), but boron concentrations on the order of approximately 0.5-1 wt % are most common.
Despite the benefit of using boron in order to produce dense, fine-grained SiC fibers and other SiC bodies, the presence of boron in SiC may have an adverse effect on some properties of SiC. For example, boron reduces the resistance to high temperature creep (i.e., relatively slow deformation under load) which may limit the utility of the SiC ceramics for applications requiring long use times under fixed loads at high temperatures, such as in engine applications. H. M. Yun, J. C. Goldsby, G. N. Morscher, and J. A. DiCarlo showed, in Paper 54 in the Proceedings of the Annual HITEMP Review 1995, NASA Conference Publication 10178, 1995, that SiC fibers with lower boron contents had better high temperature creep resistance. Studies with bulk SiC samples also indicate that boron has an adverse effect on creep resistance. The presence of boron in SiC may also have an adverse effect on other SiC properties, such as the stability and mechanical properties of SiC ceramics that are exposed to nuclear irradiation.
This invention describes a general method for the fabrication of SiC fibers, preferably with dimension of less than 50 microns, with low boron content. Alternatively, the method can be used to reduce the concentration of boron that may be present in pre-existing SiC fibers. The method allows the fabrication of fibers with boron contents less than 0.1% and tensile strengths of 2.0 GPa or greater.
The method can also be applied to other SiC carbide ceramics, such as SiC coatings, thin substrates, etc. It could also be applicable to porous bulk SiC ceramics composed of a particulate or fiber framework. In general, the method can be used to prepare SiC ceramics with low boron content or can be used to reduce the boron content in pre-existing SiC ceramics.
There are many methods of forming SiC ceramics. SiC ceramics are often prepared by forming and consolidating fine SiC particles into a desired shape and subsequently heat treating to sinter the "green" shape in order to eliminate the interparticle pores or void spaces and to obtain a strong body with high relative density and little or no residual porosity. SiC ceramics are also prepared by other methods, especially by chemical vapor deposition (CVD) and by heat treatment of organosilicon polymers. For example, organosilicon polymers have been used to prepare fine SiC particles, fibers, bulk samples, coatings, etc. Some samples prepared using organosilicon polymers develop fine SiC crystallites and fine pores during processing and, hence, a sintering step is required to produce a dense SiC sample.
The typical sintering temperatures for preparing dense SiC are in the range of approximately 1700-2300.degree. C. The required temperature is highly dependent upon the size of the SiC particles or crystallites which comprise the porous body that is being sintered. For example, SiC bodies fabricated from the more conventional powder processing routes generally require higher sintering temperatures in the range of approximately 1900-2300.degree. C. and this results in sintered bodies comprised of coarser grain sizes (greater than 1 micron). In contrast, organosilicon polymer-derived SiC bodies can be sintered at lower temperatures (e.g., from approximately 1700-1900.degree. C.) and, consequently, the resulting grain sizes are usually smaller (less than 1 micron).
One of the earliest uses of boron sintering additives was for processing SiC shapes by hot pressing or pressureless sintering of SiC powders. Prochazka in U.S. Pat. Nos. 3,853,566, 3,960,577, 3,954,483, 3,968,194, and 4,004,934 reported that small amounts of boron, especially when combined with small amounts of carbon, could be used to enhance densification and produce SiC bulk samples with high densities and fine grain sizes. Boron was incorporated by mixing SiC powders with boron-containing powders such as elemental boron, boron carbide, and boron nitride. It was also reported that gaseous boron trichloride could be used to vapor-phase dope boron directly into SiC powders during their synthesis by CVD methods. The minimum amount of boron needed for good densification was approximately 0.3 wt %, although higher amounts were generally used. Because Prochazka used powder processing methods, sintering and hot pressing temperatures were generally in the range of approximately 1900-2100.degree. C., with the lower temperatures in this range being used for samples which were hot pressed. The boron-doped SiC bodies were sintered to high relative density using several different gas atmospheres, including argon, helium, nitrogen, and vacuum.
Numerous other studies have confirmed the benefit of using boron or boron-containing compounds as sintering aids in powder processing of SiC with high relative density and fine grain sizes. The minimum required concentration of boron are reported to be at least 0.2-0.3 wt %, but for practical application the amounts of boron needed are usually in the range of from approximately 0.5-1.0 wt % or higher.
Powder processing methods can be used to form SiC fibers, especially fibers with larger diameter (i.e., greater than 25 microns). Frechette et al. in U.S. Pat. No. 4,908,340 and in Ceram. Eng. Sci. Proc. 12[7-8]992-1006, 1991 (by F. Frechette, B. Dover, V. Venkateswaran, and J. Kim) prepared SiC fibers by either melt spinning or dry spinning of mixtures of SiC particles and organic polymers. Boron was incorporated in the fibers by adding 0.2-1.0 wt % boron carbide powders to the spinning mixtures. Some fibers were heat treated in crucibles which had been coated with slurries containing boron carbide in order to form boron-containing gases during the sintering operation. SiC fibers were sintered at temperatures in the range of 1900-2150.degree. C. in an argon atmosphere or at 2300.degree. C. in a nitrogen atmosphere in order to obtain substantially dense, low porosity, fibers. It is reported in U.S. Pat. Nos. 4,004,934 and 4,908,340 and elsewhere that higher sintering temperatures are required for densification when nitrogen is used as the sintering atmosphere.
Bolt et al. in U.S. Pat. No. 4,942,011 prepared SiC fibers by spinning mixtures of SiC particles and organosilicon polymers. One of the sintering aids used was boron carbide. The concentration of the sintering aids was 0.2-5% based on the SiC weight. Fibers were heat treated at temperatures in the range of 1900-2000.degree. C. in an argon atmosphere.
Birchall and Clegg in U.S. Pat. No. 5,063,107 prepared SiC fibers in a similar manner to Frechette et al. and Bolt et al. They used mixtures of SiC particles and organic polymers. They also utilized known sintering aids, such as boron. Sintering was carried out at 2040.degree. C. for 20 minutes in an argon atmosphere.
The use of boron in gaseous form for enhancing densification of SiC was clearly demonstrated by Smoak in U.S. Pat. No. 4,237,085. SiC green compacts were initially prepared from fine SiC particles without incorporating any boron source within the compact. The green compacts were then sintered at high temperature in an atmosphere which contained boron gases. The boron gases were formed by putting volatile boron sources (e.g., boron trichloride, elemental boron, or boron carbide) in the sintering furnace. These sources formed boron-containing vapors during the high temperature sintering treatment. The diffusion and infiltration of the boron gases into the green compacts allowed densification to proceed so that high sintered densities were achieved.
Boron-doped SiC bodies with high relative density have also been formed using mixtures of SiC particles and borosiloxane polymers. This was demonstrated by Burns et al. in U.S. Pat. No. 5,112,779 using sintering temperatures in the range of 2000-2200.degree. C. to achieve densification. Suitable atmospheres for sintering were argon, helium, nitrogen, and vacuum.
The beneficial effect of using boron doping to form strong, dense SiC has also been demonstrated in other methods of preparation of SiC, including preparation methods using CVD and organosilicon polymers. It has been shown that SiC fibers prepared by CVD can be doped with boron by a vapor phase method. D. E. Elkins, G. M. Mallon, and H. Shimizu, in "Modified Silicon-Carbide Continuous Filaments," D-41-D-49, 10th National Symposium, SAMPE, San Diego, Calif., November, 1966 discovered that gaseous boron halides could be used to incorporate varying amounts of boron into SiC fibers. The boron additions resulted in fibers with smoother surfaces, finer grains sizes, and higher tensile strengths.
Organosilicon polymers are the preferred material for forming SiC fibers with fine diameters of less than approximately 25 microns. Yajima et al. in U.S. Pat. Nos. 4,052,430, 4,100,233, 4,220,600, and 4,283,376 developed processes for preparing polycarbosilane polymers and SiC-based fibers from polycarbosilane polymers. The processes developed by Yajima et al. are the basis for NICALON.TM. fibers which are manufactured by Nippon Carbon Co., Ltd. A similar commercial process involves using a titanium metal-modified polycarbosilane (i.e., a polytitanocarbosilane) polymer and is the basis for TYRANNO.TM. fibers which are manufactured by Ube Industries Ltd. Yajima et al., in some of the earlier cited patents and in U.S. Pat. No. 4,152,509, also reported that SiC fibers could be prepared from poly carbosilane polymers which were synthesized from a polysilane and a phenyl-containing polyborosiloxane. A typical method for synthesis of the boron-containing polymers (i.e., phenyl-containing polyborosiloxane polymers) involved reacting boric acid with a diphenylchlorosilane, although other reactants were also reported. The organosilicon polymer fibers produced by the various methods of Yajima et al. were pyrolytically decomposed to SiC-based fibers by heat treatment at temperatures usually less than 1300.degree. C. Pyrolysis was usually carried out in nitrogen or argon atmospheres or under vacuum. The fibers produced by these methods do not have good thermomechanical stability. The fibers degrade extensively and become extremely weak when heat treated at temperatures above approximately 1200-1400.degree. C. This is because of carbothermal reduction reactions that occur as a result of large amounts of oxygen and excess carbon in the SiC fibers.
Other methods have been reported for introducing boron into organosilicon polymers and using such polymers to prepare SiC-based fibers. Takamizawa et al. in U.S. Pat. No. 4,604,367 prepared SiC-based fibers which contained boron by using organoborosilicon polymers. These polymers were prepared by reacting an organopolysilane with an organoborazine compound, such as B-trimethyl-N-triphenyl borazine, B-trivinyl-N-triphenyl borazine, and B-triamino-N-triphenyl borazine. The organoborosilicon polymers were melt spun into green fibers which were infusibilized by oxidative or irradiative cross-linking methods and subsequently pyrolyzed to form SiC-based fiber containing boron and nitrogen. Takamizawa et al. indicated that heat treatments could be carried out in vacuum or inert gas atmospheres at temperatures up to 1800.degree. C., although the preferred heat treatment temperatures were less than 1600.degree. C. The resulting fibers showed better strength retention upon high temperature heat treatments compared to NICALON.TM. fibers, but the fibers still started to show decreased strengths after heat treatments at temperatures above approximately 1100-1200.degree. C. and the strengths were too low for the fibers to be useful at temperatures above approximately 1500.degree. C.-1600.degree. C.
Researchers at Dow Corning have reported in U.S. Pat. Nos. 5,071,600, 5,162,269, 5,167,881, 5,268,336, 5,279,780 and 5,366,943 the fabrication of boron-doped, low-oxygen-content SiC fibers with carbon-rich or near-stoichiometric composition which were prepared based on using several organosilicon polymers (i.e., polycarbosilane, methylpolydisilylazane, polyorganosiloxane). In U.S. Pat. Nos. 5,071,600 and 5,162,269, for example, fibers were formed by melt spinning of low-molecular-weight polycarbosilane. Fibers were oxidatively cured to prevent melting of the fibers during subsequent heat treatment. The primary modification of the process of Yajima et al. was to incorporate sufficient amounts of boron (greater than 0.2 wt %) in the fibers so that high relative density and fine grain sizes could be obtained after sintering. The patents describe mostly gas-phase doping methods in which it was indicated that boron-containing compounds (e.g., diborane, boron trifluoride, boron tribromide, boron trichloride, tetraborane, pentaborane, borazine, trichloroborazine) could be infiltrated into the fibers at temperatures in which the compounds were in the form of gases. The boron-containing gas could be introduced at various stages in the fiber fabrication process, but it must be present prior to the onset of sintering in order to prevent grain coarsening and to allow densification of the porous SiC fibers to occur. The fibers develop fine pores after a high temperature pyrolysis process which typically occurs in the range of approximately 1200-1600.degree. C. The porosity is removed by sintering at a temperature typically in the range of 1800-1850.degree. C. in an argon atmosphere. In addition to gas phase doping, the Dow Corning patents also describe instances in which the organosilicon polymer can be synthesized such that it contains some boron. The fibers produced according to the methods described in the Dow Corning patents had much better thermomechanical stability compared to some other organosilicon polymer-derived fibers, such as the NICALON.TM. and TYRANNO.TM. fibers. According to Lipowitz et al. (in Ceram. Eng. Sci. Proc., 16[4]55-62, 1994), the fibers retained 96% of their original tensile strength after heat treatment at 1550.degree. C. for 10 hours in an argon atmosphere. The fibers retained 74% of their original tensile strength after heat treatment at 1800.degree. C. for 12 hours in an argon atmosphere. The fibers produced according to the Dow Corning patents are known as SYLRAMIC.TM. fibers.
Sacks et al. in pending U.S. patent application Ser. No. 08/683,475 reported the fabrication of boron-doped, low-oxygen-content SiC fibers with near-stoichiometric composition, high relative density, and high tensile strength which were prepared using high-molecular-weight polycarbosilane polymers. Fibers were sintered in an argon atmosphere. In most examples, the fibers were sintered in the range of 1750-1800.degree. C. for 1 hour. If fibers were sintered at higher temperatures, shorter sintering times were used to produce fibers with high relative density and high strength. For example, sintering times were 12 and 2 minutes when sintering temperatures were 1845 and 1890.degree. C., respectively. The fibers typically retained more than 90% of the original tensile strength after being re-heated in an argon atmosphere at 1800.degree. C. for 4 hours and approximately 75% of the original strength after heat treatment in argon at 1950.degree. C. for 1 hour.